i just installed a filter to my 5gal tank with my male betta yesterday. and i saw him dead today. what could be the cause? when i just installed the filter, i saw him hiding on it's side. help please, i don't want it to happen again. do they dislike the current? or the filter itself? i do regular water changes every 3-4 days, it's a planted 5gal tank. my fish lived happily in a bowl before i put him into the tank. thanks!

hi
they do dislike the current,so unless you can turn it down,
or maybe replace where it is situated so that the jet of water can't be as strong.or place a thick plant in front of the outlet nozzel,
to break the current up.
maybe you can experiment with it now the tank is empty.

i did turn it down since i've read they prefer soft currents and not torrents.. :(
it was a steady stream of small bubbles just enough to make enough surface disturbance so there won't be any oily build-up on the surface. however, he still died :(

The corner box filter sholud not have caused the death of your betta, unless you used a 150psi compressor to drive it. My guess would be that A.) There may have been some type of lethal contaminates in or on the filter and its media, or B.) He was already ill and you may not have realized it. In either case your are now without your fish.

Some fixes for your problems, although they won't help you now they will be of use in the furture. 1.) Clean, with clean, water, no soap or detergents, all things placed into you aquarium, even filters and their media. 2.) If you add a filter, powerhead, power filter or such make sure that it does not overpower the tandk and its inhabitants. If it presents excess current, turn it down, if possible. You can also, as in your case, add plants such as elodea to help break up the current and provide a safe respit for your betta. 3.) keep a close eye on your fish. Take note of how they act. Any deviation, even the most minute, may be a sign of sickness.

i have read that filter media should be rinsed with aquarium wastewater, is this not right?

This is more referring to when the filter has become clogged and you are rinsing some of the gunk out of it.

For a new filter normal tap water will be fine.

Another idea is to place the filter so that the flow hits off a wall of the tank which makes it less poweful for the fish. I've found that bettas need a filter like any other fish in terms of tank maintanence, but they just need weak ones.